The present invention relates to an image forming apparatus of the type which copies an original at a desired magnification.
An example of the known image forming apparatus of this type is a copy machine with an enlargement/reduction function. For making clearer the disadvantages of the prior art and objects of the present invention, the principles of the copy machine with such a function will be given referring to FIG. 1.
An original Q is optically scanned by a scanning section 2 when the section 2 moves along an exposure table 1 in the arrow directions N. The scanning section 2 is provided with an exposure lamp 3 and a mirror 4. The light, reflected from the original Q when it is illuminated by the exposure lamp 3, is led to a photosensitive drum 8 through a group of mirrors 4 to 7. Disposed between the mirrors 6 and 7 is a lens 9 for variably setting the magnification of a picture of the original Q. The photosensitive drum 8, as previously charged, has an electrostatic latent image formed thereon when it is illuminated with light containing the image information. The latent image is developed with toner particles and transferred onto a paper P as fed thereto. In this way, a picture of the original Q is copied on the paper.
The magnification of an image size, as viewed in the scanning direction, is determined by a ratio (speed ratio) of a scanning speed (moving speed) of the scanning section 2 and a rotating speed of the photosensitive drum 8. A magnification of the image size, as viewed in the direction normal to the scanning direction, is determined by a ratio (optical path ratio) of the lengths of the optical paths between the original Q and the lens 9 and between the lens 9 and the photosensitive drum 8. For selecting a desired magnification of the image size, a magnification setting means is operated to properly set the speed ratio and the optical path ratio. Specifically, the magnification setting means contains a speed-ratio setting means for the moving speed of the scanning section 2 and the rotating speed of the photosensitive drum 8, and contains an optical-path ratio-setting means for changing an initial position of the lens 9 to change a length of the optical path.
In the prior art copy machine, upon inputting a desired magnification into the machine, the magnification setting means starts its operation to drive the speed-ratio setting means and the optical-path ratio-setting means. With the completion of a sequence of operations, a magnification setting operation terminates. One frequently encounters a case that an operator mistakenly inputs an undesired magnification, and soon after its setting discovers it to be undesirable. In such a case, if a desired magnification is input immediately after he discovers the mistake, the copy machine rejects the input of the new magnification until the sequence of the magnification setting operations is completed. Particularly, the optical-path ratio-setting means must be operated gently to such an extent as to give a tolerable impact to the optical system contained in the machine. Therefore, its operation takes a relatively long time.
Thus, the prior art copy machine inevitably takes a long operation time when a magnification, once set, must be reset to another magnification immediately after the old magnification has been set.